Vinyl monomers radical chain scheme

A chain reaction polymerization of vinyl monomer, which is usually carried out by a photoinitiator to produce a primary radical (R ), which can interact with a monomer molecule (M) in a propagating process to form a polymer chain composed of a large number of monomer units (see Eq. [2] and reaction Scheme [3]. 

Photoinduced free radical graft copolymerization onto a polymer surface can be accomplished by several different techniques. The simplest method is to expose the polymer surface (P-RH) to UV light in the presence of a vinyl monomer (M). Alkyl radicals formed, e.g. due to main chain scission or other reactions at the polymer surface can then initiate graft polymerization by addition of monomer (Scheme 1). Homopolymer is also initiated (HRM-). 

According to this scheme the fundamental difference in the mechanism of free-radical copolymerization of MA with TASM and of MA with alkyl acrylates is due to the fact that in the former copolymerization intermolecular coordination is involved. This coordination is similar to the effect of various complexing agents (ZnCl2, SnCft and A1C13) on free-radical homo- and copolymerization of vinyl monomers. This effect seems to favor the appearance of isotactic configurations along the main chain. 

We will now consider successively the different reactions (4) and (5) of this reaction scheme in order to examine what is their relative importance for obtaining block or graft copolymers. Initially it was admitted that the graft copolymers were produced only by reaction (4b), considering that in the competition between monomer and substrate for primary initiator radicals the addition of vinyl monomers [reaction (2)] is much easier than the abstraction of an atom of another molecule [reaction (4 a)]. This assumption is based on the higher value of the activation energy for a chain transfer reaction than for a monomer addition, at least in the case of saturated molecules (88). 

Radiation-Induced Polymerization. Polymerization induced by irradiation is initiated by free radicals and by ionic species. On very pure vinyl monomers, D. J. Metz demonstrated that ionic polymerization can become the dominating process. In Chapter 12 he postulates a kinetic scheme starting with the formation of ions, followed by a propagation step via carbonium ions and chain transfer to the vinyl monomer. C. Schneider studied the polymerization of styrene and a-methylstyrene by pulse radiolysis in aqueous medium and found results similar to those obtained in conventional free-radical polymerization. She attributes this to a growing polymeric benzyl type radical which is formed partially through electron capture by the styrene molecule, followed by rapid protonation in the side chain and partially by the addition of H and OH to the double vinyl bond. A. S. Chawla and L. E. St. Pierre report on the solid state polymerization of hexamethylcyclotrisiloxane by high energy radiation of the monomer crystals. 

Compound 35 contains a thermolabile C-C bond which, on thermally induced fragmentation, yields a high proportion of macroradicals. The PDMS diradical also acts as a counter radical and can undergo chain extension at both ends in the presence of vinylic monomers (acrylonitrile, maleic anhydride, diethylfumarate) or styrenic monomers, leading to diblock copolymers in 95% yield according to the following scheme [211, 212] ... 

Ester derivatives of benzoin are known to display usually quite a low photoinitiation activity in the polymerization of vinyl monomers [85,104]. By contrast, ben2 in alkyl ethers are claimed to generate [17], by a photofragmentation mechanism, benzoyl and a-alkoxy benzyl radicals resulting in a much more active polymerization and crosslinking initiating species (Scheme 25). Thus, polymeric systems having the above moieties in the side chains have been prepared and their photoreactivity studied in more detail [105-107],... 

Dithiocarbamate-iunctionalized polymers of styrene and MMA at both ends are prepared [148] by thermal free radical initiation with tetraethylthiuram disulfide which is known [146] to behave as initiator, chain transfer agent and terminator (iniferter). Successive photolysis of the terminal dithiocarbamate end groups, in the presence of another vinyl monomer, allows one to obtain three-block cc lymers (Scheme 43). 

In 1985, Rizzardo et al.27 filed a patent for the use of alkoxyamines (Scheme 12) as regulating initiators for the living radical polymerization and block copolymerization of vinyl monomers. R is a group that upon dissociation (Scheme 10) forms a radical that adds to the monomer. The mechanism was disclosed shortly thereafter and involves the reversible dissociations shown in Scheme 11, with the nitroxide radical taking the role of X.28 In a later simulation, the group also revealed the reason for the remarkable absence of the usual terminations and rediscovered the principles of the persistent radical effect 29 As chains undergo termination transient radicals are removed from the system and the concentration of persistent species builds . Further, the authors noted correctly that, in contrast to normal radical polymer-... 

Scheme 15 summarizes the current understanding of LCo in free radically polymerized vinylic monomers. It differs from the previously available scheme25 in that the formation of the radical adduct with LCo is not the first step of CCT but rather is a reaction that poisons the catalyst. Second, the scheme includes catalytic termination arising through the reaction of LCo1 with radical chains. Its should be emphasized that essentially all of the reactions are substantially reversible. The directions of the arrows indicate the course of productive transformations. 

This is perhaps the most well-known method of polymerization, and as the name implies, involves the continuous addition of monomer units to a growing free-radical chain.The general mechanism of this process in relation to the polymerization of a vinyl monomer is shown in Scheme 1. As Scheme 1 shows, initiation is a two-stage process in which, first a free radical is formed, and second this radical adds on to a monomer unit. The second stage is essentially the same for... 

In the Soviet study110, the following elementary stages were taken into account in the kinetic scheme of vinyl acetate polymerization chain transfer to the monomer, solvent, and polymer, and chain termination caused by the disproportionation of radicals. It was assumed that long-chain branches could be formed by chain transfer both to the acetate group hydrogen atoms and to the main chain hydrogen. 

However, it is difficult to say at present to what an extent the picture shown in Scheme 7 and Fig. 2 corresponds to reality. In any case, the free-radical polymerization of symmetric vinyl monomers is not forbidden from the standpoint of symmetry, and ethylene is known to be polymerized by the free-radical mechanism. It should be noted that in the case of quasisymmetric ethylene derivatives (e.g. propylene the quasisymmetric character of which is due to real symmetry of its frontier rc-orbitals), free-radical polymerization is sometimes interrupted as early as in the stage of oligomer formation because chain termination reactions proceed intensively [37]. This has naturally no relationship to any exclusion with respect to symmetry. 

Radical chain polymerization, as noted above, is a chain reaction which involves mainly three steps - initiation, propagation, and termination, taking place in sequence. The overall scheme of the polymerization of a vinyl or related monomer M, initiated by the decomposition of a free-radical initiator I, may be schematically represented as follows (Ghosh, 1990) ... 

The first step is the rate-limiting step in the initiation process. That the above scheme represents that initiation mechanism, and is indeed partly evidence for the free-radical mechanism in addition polymerization of vinyl monomers, has been established by the presence of initiator fragments found to be covalently bound to polymer of molecules. The efficiency with which radicals initiate chains can be estimated by comparing the amount of initiator decomposed with the number of polymer chains formed. 

Scheme 3.5 Free-radical chain transfer to monomer (vinyl acetate).

The basic reaction is a radical polymerisation starting from vinyl monomers, i.e., unsaturated molecules of form R — CH = CH2, where R is a substituent which may itself be unsaturated. The monomers may be introduced singly to produce homopolymers, but more commonly, they are used in mixtures for the preparation of copolymers. Monomer molecules are highly sensitive to the presence of free radicals R these primary radicals initiate chain reactions which add monomer units, following the classic scheme... 

Although the free-radical polymerization of MMA typically exhibits a syndiotactic bias (rr triad content = 60%-70%), it has long been known that the stereochemical interactions between the chain-end radical and vinyl monomers in free-radical polymerization can be modified by using chiral protecting groups. For example, the 80 °C AIBN-initiated polymerization (AIBN = 2,2 -azobisisobutyronitrile) of oxazolidine acrylamides based on valine and t rt-leucine ultimately yields highly isotactic (92% m dyad content) poly(acrylic acid) and PMA after chemical modification (Scheme 23.23). " ... 

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